1. Field of the Invention
This invention relates in general to electronic digital signaling systems, and more particularly to a system and method for sampling an electronic signal to convert the electronic signal to a digital representation thereof.
2. Description of Related Art
A variety of electronic devices such as computers, monitors, flat panel displays, wireless communication devices, cellular phones, high speed two-way digital transceivers, and paging devices, to name just a few, utilize a plurality of electronic signals. These include clock signals, vertical-synch and horizontal-synch signals, spread spectrum and digital wireless communication signals, etc. A predominant trend in electronic devices is the use of digital signals. As it is well known to those of ordinary skill in the art, there are many advantages to representing electronic signals in digital signal form in many such electronic devices. However, in certain situations it is necessary to communicate electronic signals using a legacy medium requiring information in an analog format. The resulting electronic signals, after being communicated across the medium, may require correction to represent the electronic signal in a digital representation thereof.
An Analog-to-Digital converter (ADC) is typically utilized to sample an analog electronic signal at a point in time and to convert the sampled electronic signal to a digital representation thereof. The ADC, in one common configuration, typically includes a resistive ladder network electrically coupled to a plurality of comparators that are respectively referenced to a plurality of reference voltages. The ADC compares the voltage amplitude of an input signal to the plurality of reference voltages and provides an output signal that is a digital representation of the input signal at a point in time. A Digital-to-Analog Converter (DAC), on the other hand, normally converts a digital representation of an electronic signal to an analog electronic signal. Utilizing the DAC to convert a sequence of digital representations to analog representations of an electronic signal at sequential points in time can provide a continuous analog electronic signal.
In one particular example, a computer graphics controller using frame buffer data transmits a digital video signal to a DAC module to provide an analog video signal at an output of a computer graphics interface. This output video signal is utilized to drive a video monitor. The video signal is coupled to an interface of a video monitor typically via a cable. The transmission via the cable medium tends to pick up noise signals and adds different forms of distortion to the analog video signal. For example, besides distortion due to capacitive and inductive effects of the cable medium, this distortion may also include jitter from the output of the computer graphics interface. On the other side of this cable medium, when receiving the analog video signal including all the noise and distortion signals, a video interface for the video monitor couples the analog video signal to a ADC module. However the graphics controller clock is not transmitted to the video monitor. For a digital video monitor, this clock must be regenerated and the sample phase adjusted to synchronize the ADC samples with the original graphics controller digital clock period. Regrettably, conventional ADC implementations have not been very successful at removing most of the noise and distortion signals from the pure analog video signal. The resulting digital representation of the video signal may include some of the noise and distortion signals, which are particularly detrimental to the quality of the video image if the clock regeneration and phase adjustment are inaccurate.
Conventional implementations of video signal reconstruction have attempted to reconstruct a digital representation of an analog video signal as follows. An ADC module is driven with a sampling clock signal to sample points in an analog video signal to identify the leading and trailing edges of any signal transition in an analog video signal. The edges of a signal transition normally are not desirable sampling points for sampling the voltage amplitude of the particular video pixel (picture element). It is desirable to sample the signal in the flat region (between the edges) of the signal transition where the voltage level is stable and may be better determined from the sample point. Prior art methods drive the ADC to sample at a point in the signal that is just before the trailing edge of the signal, where the flat region was expected to be most likely stable. This sampling point is selected simply to avoid the leading and trailing edges. However, any clock jitter, for example, tends to defeat this sampling method because it is very difficult to select a sampling clock rate that avoids the trailing edge of the signal transition while intermittent jitter keeps moving the trailing edge of a signal transition relative to a time reference. Additionally, other sources of noise may be present during the flat region of the signal and a sample taken by the ADC during this noise signal will possibly provide a false measurement of signal level.
Unfortunately, these simplistic methods of driving the ADC to avoid the signal edges have not been very successful and the noise and distortion signals tend to pass through and be included into the digital representation of the video signal. This reduces the quality of a video image, and leads to the loss of image information, that is presented via the video monitor display to a user. The result is a lower opinion of the quality of the video monitor system and reduced commercial viability of products. This reduced quality and lost information can impact other applications as well. For example, distorted or lost information in a wireless communication signal can significantly impact or even destroy a wireless communication.
Thus, there is a need to overcome the disadvantages of the prior art as discussed above, and in particular to improve the quality of conversion of the analog electronic signal to a digital representation thereof.